<!doctype>
<html lang="en">
<head>
  <style>
button {
  background-color: hsl(180, 50%, 50%);
  border: 0;
  color: white;
  font-size: 1rem;
  padding: .3em 1em;
  transition: border-radius 0.5s ease,
              background-color 1s linear;
}
button:hover {
  background-color: hsl(0, 50%, 50%);
  border-radius: 1em;
}


.flex {
  margin: 0 -1em;
}
.flex > * {
  margin: 0 1em;
}

  </style>
</head>
<body>
  <button>Hover over me</button>
</body>
</html>
